RegVac has to open each time that you start your computer so that it can check to see if it has been a month yet. When it sees that it has not been a month, it closes.
If you do not want that to happen, you will have to use a different means to run RegVac. There are command-line parameters that can be used when starting RegVac from another program. RegVac's Help tells about the command-line parameters.
If you select "Do Not Perform" from "Run at Startup", RegVac will remove itself from the Startup folder. If you have something other than "Do Not Perform" selected, RegVac will be in the Startup folder. This is normal.